1. Field of the Invention
The present invention relates to a tooth used for a jaw and tooth model, using which students aspiring to become dentists can experience intraoral work and perform training related to treatment. A tooth for a jaw and tooth model is a tooth which is used for simulation of an intraoral remedial procedure or training related to treatment using a jaw and tooth model in a university. More particularly, the present invention relates to a tooth which is used to experience formation of an abutment tooth, formation of cavity or the like by grinding the tooth, and a method for producing the same.
2. Description of the Related Art
In training related to dental treatment, teeth extracted from human bodies and animals have conventionally been used so as to experience the grinding feel of a natural tooth. However, there are hygienic problems associated with extracted teeth and infection may occur if hygiene control is not sufficiently conducted. Thus it was impossible to conduct training freely. Furthermore since extracted teeth comprise a natural living material, there arises a problem of corruption and it is necessary to pay a careful attention to storage.
Therefore, there has been a need for a method which enables the experience of grinding feel of a tooth without using a natural tooth.
Now a tooth for a jaw and tooth model, which is used for training related to intraoral treatment, is often produced using an epoxy resin or a melamine resin, and are well known in the art.
However, a tooth for a jaw and tooth model formed of an epoxy resin or a melamine resin is in the form of a natural tooth, but exhibits grinding feel which is different from that of a natural tooth. Therefore, even when performing training related to the formation of an abutment tooth and formation of cavity, grinding feel and working properties are different from those of actual intraoral work, and thus the training effect could not be obtained.
Specifically, a natural tooth comprises tooth enamel and dentin, and tooth enamel and dentin are harder than resin, whereas, epoxy resin and melamine resin are soft. Thus there is a tendency to over grind and even when performing training using such a model tooth, the grinding may not be similar to that of a hard natural tooth.
Furthermore, tooth enamel and dentin, with which a crown portion of dentin is coated, have different hardnesses. As a result, in case of a transition during grinding from tooth enamel to dentin, the dentin may be excessively ground and thus a tooth may not be satisfactorily produced.
As a result of the need for a harder material, a composite type tooth is commercially available. However even in case of a composite type tooth, since the dentin portion and the enamel portion exhibit the same grinding feel, the grinding feel of the tooth is different from that of a natural tooth. Therefore, even when performing training related to the formation of an abutment tooth and the formation of a cavity, the grinding feel and working properties are different from those in case of intraoral work in practice, and thus the training effect could not be obtained. To state the matter clearly, the grinding feel includes a sliding sensation and is substantially different from a natural tooth.
That is, it is required that the grinding feel varies similarly to a natural tooth in the transition portion from the enamel portion to the dentin portion of a tooth for a jaw and tooth model. Therefore a mater of course, it is important that the enamel portion reproduces a grinding feel of tooth enamel, while the dentin portion reproduces a grinding feel of dentin.
Japanese Unexamined Utility Model Publication (Kokai) 1-90068 discloses that a tooth enamel layer is formed of glass/ceramics having a Vickers hardness controlled within a range from 350 to 450, comprising a phlogopite crystal [NaMg3(Si3AlO10)F2] and a lithia-alumina-silica-based crystal (Li2O.Al2O3.2SiO2, Li2O.Al2O3.4SiO2) precipitated simultaneously; a tooth root layer is prepared in advance by adding white, red and yellow colorants to a polyol (base resin), mixing with an isocyanate prepolymer (curing agent), injecting the mixture into a silicone rubber mother mold under vacuum conditions and curing the mixture at a normal temperature; and a dentin recognition layer, which exists between the tooth enamel layer and the tooth root layer thereby bonding both layers, and is formed of an adhesive resin having an opaque color.
However, the tooth comprising a tooth enamel layer formed of a phlogopite crystal or a lithia-alumina-silica-based crystal is not well adapted for use because it feels too hard when grinding as compared with a natural tooth, and also it is not well adapted for use because the dentin recognition layer is formed of an adhesive resin which feels too soft when grinding.
Furthermore, dentin layer is formed of an adhesive layer is disclosed. It is described that an enamel layer portion and a tooth root layer portion are formed and bonded. It is recognized as the dentin layer formed of a thick adhesive layer.
Japanese Unexamined Patent Publication (Kokai) No. (hereinafter referred to briefly as “JP-A-”) 5-224591 discloses a tooth model which has grinding properties extremely similar to those of a natural tooth, and is suited for the training of practical dental grinding.
The tooth model comprises a crown portion whose surface has a Knoop hardness of at least 70 or more and a tooth root portion whose surface has a Knoop hardness of at least 10 to 40. The tooth model contains, as main components, an inorganic matter powder and a crosslinking resin in a weight ratio of 20%:80% to 70%: 30%.
The above prior art discloses that “a tooth model may be formed of a raw material having any hardness, for example, metal, ceramics or a resin, or may be a cavity in view of the method for producing a tooth model and economic considerations”. However, this is not a tooth model which can exhibit a difference in grinding properties between the enamel portion and the dentin portion.
JP-A-5-216395 discloses a tooth model which has grinding properties extremely similar to those of a natural tooth and is suited for the training of practical dental grinding, and a method for producing the same. The tooth model contains, as main components, a hydroxyapatite powder having a porosity of 40 to 80% and a (meth)acrylate ester-based resin in a weight ratio of 20%: 80% to 50%: 50%. However, this is not a tooth model which can exhibit a difference in grinding properties between the enamel portion and the dentin portion.
JP-A-5-241498, JP-A-5-241499 and JP-A-5-241500 describe an inorganic filler and a hydroxyapatite filler. However, in all tooth models disclosed in these publications, a resin is used as a base material and thus the problem of the grinding feel is not solved. They are not tooth models which can exhibit a difference in grinding properties between the enamel portion and the dentin portion.
JP-A-2004-94049 describes an invention which provides a model tooth for dental training, which enables an accurate shape measurement with laser beam.
The specification discloses that “known materials can be used as a material constituting a surface of a crown portion of the model tooth of the present invention and there can be used porcelain materials such as ceramics; thermoplastic resin materials such as acryl, polystyrene, polycarbonate, an acrylonitrile-styrene-butadiene copolymer (ABS), polypropylene, polyethylene, and polyester; thermosetting resin materials such as melamine, urea, unsaturated polyester, phenol, and epoxy; and materials obtained by adding various organic and inorganic reinforcing fibers (for example, glass fiber, carbon fiber, pulp, synthetic resin fiber, etc.), various fillers (for example, talc, silica, mica, calcium carbonate, barium sulfate, alumina, etc.), colorants (for example, pigment, dye, etc.) and various additives (for example, weathering agents, antistatic agents, etc.) to these main raw materials”. However, there is no description about preferable materials, and thus the problem of grinding feel is not solved.
As a result of a study, the present inventors have found that it is necessary to use a sintered body of an inorganic material so as to exhibit the grinding feel of a natural tooth. Because of the difficulty in controlling a hardness of an inorganic material, it is difficult to form an enamel portion and a dentin portion while controlling the hardness.
In particular, when the enamel portion and the dentin portion of the tooth model are formed of a sintered body of the same inorganic material, it was difficult to reproduce a difference in the grinding feel between the enamel portion and the dentin portion.
It is proposed that the enamel portion and the dentin portion are separately formed and a grinding feel of each portion is adjusted so as to reproduce a difference in grinding feel between the enamel portion and the dentin portion.
It is necessary to unify the density, particle shape and firing temperature of the sintered body so as to adjust the grinding feel of the sintered body. The difference in a shrinkage rate and a thermal expansion coefficient between the enamel portion and the dentin portion upon firing results in breaking, peeling and cracking. In addition, a gap may be formed between the dentin portion and the enamel portion. Thus, chipping may occur upon grinding and the gap gives a feel different from the grinding feel of a natural tooth. Thus the resultant tooth was not well adapted for use.
In particular, when the enamel portion and the dentin portion are formed as separate sintered bodies, adhesion between the enamel portion and the dentin portion is required and grinding feel largely varies in an interfacial region including the adhesion portion. That is, there is obtained a tooth model which causes a strange sensation during the transition from the enamel portion to the dentin portion, and which is very difference from a natural tooth.
When a natural tooth is ground, a unique tough grinding feel upon grinding of a living body is obtained. Although various methods have been studied so as to obtain a tough grinding feel peculiar to a natural tooth, sufficient grinding feel cannot be obtained using resin, composite or the like and such a sensation can not be obtained while pouring water on a conventional tooth for a jaw and tooth model. A grinding feel is required in which users feel a tougher grinding feel than that of an inorganic material even in case of the tooth enamel because of a similar phenomenon.
In view of the above, a conventional tooth model is not satisfactory with respect to grinding properties and, therefore, it is required to develop a tooth model having grinding properties similar to those of a natural tooth. However, the tooth models have not exhibit a sufficient grinding feel. It is particularly required to develop a tooth having toughness peculiar to a natural tooth, and there has never been obtained a tooth model capable of exhibiting a difference in grinding properties between the enamel portion and the dentin portion.
However, neither a specific composition of a tooth model capable of realizing grinding feel of tooth enamel and dentin of a natural tooth, nor a method for producing the same has been studied or reported.
Therefore, it is required to develop a tooth model, the enamel portion and the dentin portion being formed of the same inorganic material, which exhibits a tough grinding feel peculiar to a natural tooth and which also reproduces a difference in grinding feel between the enamel portion and the dentin portion.
A method of reproducing tooth pulp peculiar to a natural tooth has not been developed heretofore, and thus dental students could not experience exposure to tooth pulp. Dental pulp exposure (grinding down the tooth pulp portion) is the most important technique in a dental treatment. In case where tooth pulp exposure was carried out by mistake, the subsequent treatment method must be learned at the same time.
As a dental caries progresses in a natural tooth, the treatment position expands into the enamel layer, the dentin layer and the tooth pulp, and training for a root canal treatment such as a pulpectomy is most important. When a pulpectomy is carried out, since the tooth pulp is removed by a reamer and sensation of rubbing of the dentin wall surface with the reamer completely varies, training related to root canal filling could not be carried out.
In a treatment of tooth pulp, a tooth designed for training of a root canal treatment is used, and also training of a root canal treatment (root canal cleaning, root canal extension, etc.) is carried out using a tooth with a small hole formed of a box-shaped acryl. However, sufficient training cannot be carried out since it is impossible to mount the tooth on a jaw and the hardness of dentin varies.
It is required to develop a tooth for a jaw and tooth model, which enables these experiences. In particular, dental students learn by hand a sensation of whether or not tooth pulp is completely removed upto an apical foramen during root canal cleaning, and thus it is difficult for beginners. Therefore, it is required to perform training using a tooth for a jaw and tooth model in which tooth pulp in a natural tooth is reproduced.
Although removal of dental caries is an important procedure in a dental treatment, a carious dental portion is softer than a usual dentin portion and thus it is difficult to grind the carious dental portion. Therefore, it is necessary to perform training using a tooth for a jaw and tooth model in which dental caries in a natural tooth are reproduced. It is also required to develop a method of confirming that a carious dental portion has been accurately removed.